Epilepsies constitute a diverse collection of brain disorders that affect about 3% of the population at some time in their lives (Annegers, 1996). An epileptic seizure can be defined as an episodic change in behaviour caused by the disordered firing of populations of neurons in the central nervous system. This results in varying degrees of involuntary muscle contraction and often a loss of consciousness. Epilepsy syndromes have been classified into more than 40 distinct types based upon characteristic symptoms, types of seizure, cause, age of onset and EEG patterns (Commission on Classification and Terminology of the International League Against Epilepsy, 1989). However the single feature that is common to all syndromes is the persistent increase in neuronal excitability that is both occasionally and unpredictably expressed as a seizure.
A genetic contribution to the aetiology of epilepsy has been estimated to be present in approximately 40% of affected individuals (Gardiner, 2000). As epileptic seizures may be the end-point of a number of molecular aberrations that ultimately disturb neuronal synchrony, the genetic basis for epilepsy is likely to be heterogeneous. There are over 200 Mendelian diseases which include epilepsy as part of the phenotype. In these diseases, seizures are symptomatic of underlying neurological involvement such as disturbances in brain structure or function. In contrast, there are also a number of “pure” epilepsy syndromes in which epilepsy is the sole manifestation in the affected individuals. These are termed idiopathic and account for over 60% of all epilepsy cases.
Idiopathic epilepsies have been further divided into partial and generalized sub-types. Partial (focal or local) epileptic fits arise from localized cortical discharges, so that only certain groups of muscles are involved and consciousness may be retained (Sutton, 1990). However, in generalized epilepsy, EEG discharge shows no focus such that all subcortical regions of the brain are involved. Although the observation that generalized epilepsies are frequently inherited is understandable, the mechanism by which genetic defects, presumably expressed constitutively in the brain, give rise to partial seizures is less clear.
The idiopathic generalized epilepsies (IGE) are the most common group of inherited human epilepsy and do not have simple inheritance. Two broad groups of IGE are now known—the classical idiopathic generalized epilepsies (Commission on Classification and Terminology of the International League Against Epilepsy, 1989) and the newly recognized genetic syndrome of generalized epilepsy with febrile seizures plus (GEFS+) (Scheffer and Berkovic, 1997; Singh et al., 1999).
The classical IGEs are divided into a number of clinically recognizable but overlapping sub-syndromes including childhood absence epilepsy, juvenile absence epilepsy, juvenile myoclonic epilepsy etc (Commission on Classification and Terminology of the International League Against Epilepsy, 1989; Roger et al., 1992). The sub-syndromes are identified by age of onset and the pattern of seizure types (absence, myoclonus and tonic-clonic). Some patients, particularly those with tonic-clonic seizures alone do not fit a specifically recognized sub-syndrome. Arguments for regarding these as separate syndromes, yet recognizing that they are part of a neurobiological continuum, have been presented previously (Berkovic et al., 1987; 1994; Reutens and Berkovic, 1995).
GEFS+ was originally recognized through large multi-generation families and comprises a variety of sub-syndromes. Febrile seizures plus (FS+) is a sub-syndrome where children have febrile seizures occurring outside the age range of 3 months to 6 years, or have associated febrile tonic-clonic seizures. Many family members have a phenotype indistinguishable from the classical febrile convulsion syndrome and some have FS+ with additional absence, myoclonic, atonic, or complex partial seizures. The severe end of the GEFS+ spectrum includes myoclonic-astatic epilepsy.
In GEFS+ families, linkage analysis on rare multi-generation large families with clinical evidence of a major autosomal dominant gene have demonstrated loci on chromosomes 19q and 2q. Both the 19q and 2q GEFS+ loci have been confirmed in independently ascertained large families, and genetic defects have been identified. Families linked to 19q are known and a mutation in the gene for the β1 subunit of the neuronal sodium channel (SCN1B) has been identified (Wallace et al., 1998). This mutation results in the loss of a critical disulphide bridge of this regulatory subunit and causes a loss of function in vitro. Families linked to 2q are also known and mutations in the pore-forming α subunit of the neuronal sodium channel (SCN1A) have been identified (PCT/AU01/01648; Escayg et al., 2000).
Severe myoclonic epilepsy of infancy (SMEI) is classed as an epileptic syndrome that manifests as both generalised and focal (partial) seizures (Commission on Classification and Terminology of the International League Against Epilepsy, 1989). SMEI begins with prolonged febrile and afebrile hemiclonic and generalised seizures in the first year of life. Between one and four years, other seizure types evolve including myoclonic, absence and atonic seizures. Neurological development is normal in infancy with progressive slowing after two years. A family history of epilepsy and/or febrile seizures is often found in SMEI patients and recent work has shown that family members have epilepsy phenotypes consistent with the GEFS+ spectrum (Singh et al., 2001; Veggiotti, 2001). From a clinical perspective, as GEFS+ and SMEI involve fever-related seizures, it was thought that sodium channel genes may be the target for mutations in SMEI affected individuals. This fact was later confirmed when mutations in the SCN1A gene in SMEI patients were identified (Claes et al., 2001; Ohmori et al., 2002). Of interest is that each of these mutations were de novo, a fact difficult to reconcile based on the clinical experience that a significant number of SMEI cases have a family history of GEFS+.
The development of a molecular diagnostic test to aid in the early diagnosis of SMEI is important. Such a test would direct the correct treatment strategy for patients likely to be affected with SMEI and would predict a risk for seizure aggravation as a result of factors such as fever induced by vaccination or other causes. Clinical studies to determine the molecular basis of SMEI have been variable in their results and have been inconclusive as to a single molecular basis for SMEI, particularly as alterations in the SCN1A gene are involved in other epilepsy subtypes. The inventors have recognised the need for such a predictive diagnostic test for SMEI and have therefore established a method that overcomes the limitations identified in previous clinical studies and determines the likelihood that an epilepsy patient has SMEI based on a molecular analysis of the SCN1A gene.